The CWE-787 vulnerability at line 13 is caused by a potential buffer overflow when converting a wide character string (`wchar`) to a UTF-8 encoded string (`utf8`).

The `ConvertWCharNToUtf8Alloc` function allocates memory for the resulting UTF-8 string based on the length of the input wide character string (`wchar`). However, the length of the resulting UTF-8 string can be longer than the length of the input wide character string, especially when dealing with non-ASCII characters.

The issue arises because the `*pLength` variable, which represents the length of the input wide character string, is not adjusted to account for the potential increase in length during the conversion process. As a result, the allocated buffer for the UTF-8 string may be too small to hold the entire converted string, leading to a buffer overflow.

To fix this vulnerability, the code should ensure that the allocated buffer is large enough to hold the entire converted string. This can be achieved by:

1.  Calculating the maximum possible length of the resulting UTF-8 string based on the length of the input wide character string.
2.  Allocating a buffer of sufficient size to hold the resulting UTF-8 string.
3.  Passing the allocated buffer and its size to the `ConvertWCharNToUtf8Alloc` function.

Here's an example of how the code can be modified to fix the vulnerability:

```c
// Calculate the maximum possible length of the resulting UTF-8 string
UINT32 utf8_len_max = *pLength * 4; // 4 bytes per wide character in the worst case

// Allocate a buffer of sufficient size to hold the resulting UTF-8 string
char* utf8 = (char*)malloc(utf8_len_max + 1); // +1 for null-termination

if (utf8 != NULL) {
    // Convert the wide character string to a UTF-8 encoded string
    UINT32 utf8_len = 0;
    if (ConvertWCharNToUtf8Alloc(wchar, *pLength, &utf8_len) != 0) {
        // Handle conversion error
    }

    // Ensure the allocated buffer is large enough to hold the resulting UTF-8 string
    if (utf8_len > utf8_len_max) {
        // Handle buffer overflow error
    }

    // Use the resulting UTF-8 string
    // ...

    // Free the allocated buffer
    free(utf8);
}
```

By allocating a buffer of sufficient size and checking for potential buffer overflows, the modified code fixes the CWE-787 vulnerability and ensures the safe conversion of wide character strings to UTF-8 encoded strings.